not a bad start
You got all the baseline functionality there.
Its fully working, and nice and stable.
You really need to think up some features to make your game stand out though. There is sooooo many tower defence games out there, you need your own little gimick. Maybe also a story, missions, or...well...somethine else.
Graphics naturaly need work, but are functionality, and I can see your not focusing on that yet. (which is a good thing, best not to spend ages on graphics untill your gameplay is pinned down).
If you can't do detailed sprite-work yourself though, you might want to take a more stylish approach to the graphics. (like a sketchy-look, or chalk, crayon etc...non realistic styles can look great and take far less effort).
Overall keep up the good work. Look forward to seeing the game develop more.